The first documented outbreak of NiV began in September 1998 in the village of Ampang in the Kinta district. A small group of index farms have been identified where several cases of fatal febrile encephalitis in humans followed respiratory and encephalitic disease in pigs. The farms in the village are interspersed with orchards containing durian (Durio zibethinus), ketapang (Terminalia catappa), rambutan (Nephelium lappaceum), ciku (Manilkara achras), and Jambu air (Eugenia aquea) trees. Many frugivorous bat species feed in these orchards, but in 1997, villagers noted the sudden appearance of Pteropus vampyrus roosting 20 km away in the forest. They foraged on D. zibethinus flowers and nectar around the village (Chua 2002 and pers. comm.).
The
outbreak spread to the Ulu Piah and Tambun areas near the town of Ipoh,
Perak (see map).1 Cases continued until February 1999 (MMWR 9 Apr 1999). The
second outbreak occurred in December 1998 and January 1999 in Sikamat, Negri
Sembilan. The third and largest outbreak in Bukit Pelandok, Negri Sembilan,
lasted from December 1998 to April 1999. Two cases occurred in the state of
Selangor, between Perak and Negri Sembilan (MMWR 9 Apr 1999). The transport
of infected pigs was accelerated by a “fire sale” that moved grower pigs
from Perak to Negri Sembilan, Selangor, Penang, Malacca, and Johore. Chua
(pers. comm.) wrote that the fire sale “occurred mainly from a farm
previously owned by Lee,” whose son died from NiV. Soon after selling the
farm, the nearby river flooded and the new owner sold the pigs at a deep
discount. Apparently, farms located immediately adjacent to affected farms
(which had received infected grower pigs) tended to escape infection if
culling was promptly undertaken. Grower pigs traditionally live only 6 mos.;
farms receiving infected breeder pigs generally became affected very
quickly. Dogs, cats, and shared boar semen may have been responsible for
inter-farm transmission within villages (Mohd Nor et al. 2000a). Dogs and
cats may also have helped transmit virus mechanically (Mohd Nor et al.
2000b).
The outbreak was arrested by a movement ban and a mass cull, which began on 20 March and ended 26 April 1999. Peninsular Malaysia had 2.4 million pigs and 1800 pig farms in January 1999; after July 21, 1.1 million pigs and 956 farms had been destroyed and 48 more had closed0 (Mohd Nor et al. 2000b). Because of initial misdiagnosis, the pigs had been vaccinated against JE several weeks earlier; the reuse of syringes may have contributed to the spread of disease within or even between farms (Chua pers. comm., Farrar 1999). The effects of the cull were hampered by farmers who, feeling inadequately compensated by the government, continued to smuggle pigs from their properties (Ai 2000). Over 2000 policemen and soldiers killed 1.1 million pigs using a cull radius of 5-10 km around recognized outbreak areas (MMWR 9 Apr 1999, Vadivale 15 Apr 1999). The WHO declared the outbreak “virtually over” in May 1999 (Anon 10 Apr 2000), and the last human fatality from acute infection occurred on 27 May 1999 (Chua pers. comm.). Pig farming is now banned in Negri Sembilan (Ai 2000).
Another outbreak occurred in March 1999 at a Singaporean abattoir. Symptoms appeared between March 9 and 19. Eleven became infected and one died. Two of the 11 had respiratory signs and nine had encephalitis (Paton et al. 1999). Four were directly involved in pig slaughter, two collected pig blood for sale, one removed internal organs after slaughter, and the remaining four herded pigs around the abattoir before slaughter. No workers from Singapore’s other abattoir, which also imports pigs from Malaysia, became sick. Workers from the infected abattoir did not report unusual illnesses in imported pigs, which are inspected before and after slaughter, but the AAHL detected HeV [sic] antibodies in blood samples from four out of 100 pigs in the other abattoir. A serological survey of 58 of the 655 affected abattoir’s workers found two testing positive to anti-NiV IgM and IgG, though they had been asymptomatic (Chew et al. 2000). There were no secondary cases among family members or contacts (Paton et al. 1999). On 19 March 1999 Singapore banned the importation of pigs from Malaysia and closed the abattoirs (Chew et al. 2000).
In 1998 and 1999, there were a total of 265 confirmed human cases of NiV and 105 deaths; the vast majority of cases in humans occurred in March (Chua et al. 2000). Serologic studies indicate that two polo ponies were infected near Ipoh (Chua et al. 2000, Anon. 8 May 1999). NiV infection was demonstrated histochemically in two dogs, a cat, and an undefined number of goats (Hooper et al. 2001, Lam and Chua 2002). Uppal (2000) reports that sheep were also infected, but Dr. Chua could not confirm infection in sheep (pers. comm.).
NiV has been retrospectively diagnosed as the cause of illnesses and one death in pig farm workers in the Kinta District of Perak in 1997 (Farrar 1999, Mohd Nor 1999). The Institute of Medical Research of the Ministry of Health tested serum samples taken from Japanese encephalitis patients admitted to Ipoh General Hospital in 1997 and found six positive for NiV antibodies (Chua pers. comm.). Mohd Nor (1999) mentioned that NiV may have been responsible for some pig deaths in 1996; Field (pers. comm.) believes the Dr. Daniel at the AAHL demonstrated the presence of NiV histochemically in a formalin-fixed tissue sample taken from a pig near Ipoh in 1996.
In 2000, the state quarantined 15 pig farms, imposed movement restrictions, and culled hundreds of pigs (Ahmad 2000). Five of the quarantined farms were in Penang, four in Perak, three in Malacca, two in Selangor, and one in Kedah (Anon 30 Jun 2000). A farm in Sarawak was false-positive under ELISA test, confirmed by serum neutralization (Chua pers. comm.).
SOURCE: MMWR 30 April 1999
1 Unlike the HeV outbreaks, which probably represent three separate transmission events from bats to domestic animals, the NiV outbreak seems to have been caused by one spillover from bats to pigs and the subsequent transport of infected pigs southward (Chua et al. 2000). Researchers have been calling the local epidemics resulting from pig movement separate outbreaks, and this review follows the same custom.
Author: S. Cobey.